Our invention is directed to a process for the hydrodesulfurization of hydrocarbon stocks employing a catalyst comprised of Group VI and Group VIII hydrogenating components promoted with a minor amount of a Group IV-B metal. More particularly, our invention is advantageously employed for the hydrogen treatment of naphtha, distillate, lubricating oil and residual stocks in the presence of aforementioned catalyst.
It has previously been suggested in the art to effect sulfur removal from hydrocarbon stocks by subjecting them to treatment with hydrogen in the presence of elevated temperature and pressure while in contact with a catalyst containing hydrogenating components, either supported or unsupported. Typical of the catalysts suggested by the prior art are those containing Group VI and Group VIII metals, or their oxides and sulfides, as the hydrogenating components, supported on a variety of well-known carriers, such as, for example, alumina, kieselguhr, zeolitic molecular sieves and other materials having high surface area. While these previously suggested techniques have generally been effective, to a greater or lesser extent, it is still desirable to obtain a hydrodesulfurization process wherein the overall operation is more effective or more advantageous from an economic viewpoint.
Our invention is directed to an improved process for the hydrodesulfurization of petroleum stocks such as naphtha and particularly those hydrocarbon stocks boiling generally above about 300.degree. F. The process of our invention comprises contacting the hydrocarbon stock with hydrogen and a substantially phosphate-free catalyst comprising Group VI and Group VIII hydrogenating components present as the metals, their oxides or sulfides, supported on a non-zeolitic carrier, which catalyst is promoted with a minor amount of a Group IV-B metal.
The feed stocks suitable for employment in our process include all naphtha and heavier hydrocarbons. The feeds particularly suitable are those containing a substantial quantity of components, i.e., greater than 50% by volume, boiling above about 400.degree. F. and preferably above about 500.degree. F. Such materials can be synthetic crude oils such as those derived from shale oil, tar sands and coal or full petroleum crudes or any individual fraction thereof. Thus, for example, our feed stock can be an atmospheric topped crude or it can be a vacuum residual fraction boiling substantially above 950.degree. F. Similarly, it can be a naphtha or any of the intermediate distillate fractions, such as, a furnace oil boiling from about 450.degree. to about 650.degree. F. or a gas oil boiling from about 650.degree. to about 950.degree. F. Preferably, however, we employ a feed stock which contains a substantial quantity of residual components, asphaltic contaminants and metalliferous components. Generally, we find that our process becomes more advantageous in the treatment of stocks wherein such components, contaminants and compounds comprise an increased proportion of the total charge stock. Accordingly, we find our process to be particularly advantageous in the treatment of residual fractions boiling substantially above 950.degree. F.
In this connection, we intend the terms "residual", "residue" or "residual components," when used herein to describe the most difficultly vaporizable portion of the crude oil which normally cannot be distilled, in the absence of a vacuum, without effecting decomposition of the stock. Indicative of such residual component in a Conradson Carbon Number usually greater than about 1. Such a residual fraction can typically be obtained by vacuum distillation, i.e., a vacuum tower bottoms.
As mentioned above, the catalyst employed in our process must contain substantially no phosphates. While the presence of phosphates in the catalyst can be tolerated on the contaminant level, i.e., less than about 0.5% by weight and preferably less than about 0.1% by weight, it is desired that no phosphates be present at all. We find that phosphate levels even as low as about 1% by weight have an adverse affect upon the catalytic activity and a phosphate content approaching 2% by weight is completely unacceptable.
The carrier or support employed in the catalyst can be any non-zeolitic refractory oxide having a surface area in excess of 3 m.sup.2 /g such as pure alumina, a so-called silica stabilized alumina containing up to about 5% by weight based upon the carrier of silica, silica gels, acid leached borosilicate glass and spinels, e.g. magnesium aluminate. Preferably, however, we employ an alumina carrier which is silica-free. Additionally, we prefer the carrier to be substantially free from the incorporation therein of refractory metal oxides, other than alumina, such as, thoria, boria, titania, magnesia, zirconia, etc., although the Group IV-B metals are to be added to the total catalyst. In any event, the preferred alumina employed in our process is not a zeolite but rather is of the more traditional type sometimes termed an amorphous alumina.
The hydrogenating component employed with our catalyst can be one of or a combination of the Group VI and Group VIII metals of their oxides or sulfides. We prefer to employ catalysts containing a combination of Group VI and Group VIII metalliferous components and particularly we prefer to employ such components in an atomic ratio of Group VIII metal to Group VI metal of at least 1:0:3, preferably at least about 1:0.5 and more preferably at least about 1:1.0. Generally, we do not employ such catalysts with a Group VIII to Group VI atomic ratio in excess of about 1:5, preferably an atomic ratio of less than about 1:3.5, and more preferably an atomic ratio of less than about 1:2.5. We find a particularly preferred catalyst contains the Group VIII and Group VI metals in an atomic ratio of less than about 1:1.75. Further, the catalysts of our invention have a total Group VI plus Group VIII metals content of at least about 5% by weight based upon the total catalyst and preferably at least about 8% by weight. As a general rule, we do not employ catalysts containing more than about 30% by weight metals and usually restrict total Group VI and Group VIII metal content to less than about 20% by weight. Preferred catalysts for use in our process can be comprised of combinations of the iron group metals and Group VI metals such as molybdenum and tungsten. Of the iron group metals we prefer to employ cobalt and nickel, with nickel being particularly preferred, and of the Group VI metals we prefer to employ molybdenum. Further, we prefer not to use chromium in the absence of other Group VI metals. Illustrative of particularly preferred catalysts for use in our invention are combinations of nickel-molybdenum and cobalt-molybdenum.
It is also a requirement of our invention that the catalyst employed be promoted with a Group IV-B metal, i.e. titanium, zirconium or hafnium. The Group IV-B component can be present in the catalyst composition as the metal, its oxide or its sulfide. Accordingly, we employ catalysts containing at least 1% by weight Group IV-B metal based upon the total catalyst and preferably containing at least about 2.5% by weight. While there does not appear to be any upper limit on the maximum amount of Group IV-B metal which can be employed, there does not appear to be any advantage to employing more than about 10% by weight based upon the total catalyst of such metal. Preferably, we employ catalysts containing less than about 8% by weight Group IV-B metal. Of the Group IV-B metals (titanium, zirconium and hafnium), we prefer to employ titanium and zirconium with titanium being particularly preferred.
The catalysts employed in our process can be produced by any of the techniques well known to the art so long as such techniques comply with the criteria set forth above. Thus, for example, a technique which would result in the incorporation of titanium into the body of the carrier, such as, for example, dispersion through or precipitation in the gel or sol precursor would not be the most preferred technique. In fact, the Group IV metal required by our invention is preferably added to the catalyst after the alumina carrier has been calcined. In this connection, we prefer to add the Group IV-B metal by the technique of impregnating the calcined alumina with an aqueous solution of a salt of the Group IV-B metal such as titanium tetrachloride.
In the preparation of the catalyst composition, the Group VI metal can be deposited on the support from an aqueous solution of a salt such as ammonium molybdate, ammonium paramolybdate, molybdenum pentachloride or molybdenum oxalate. After filtering and drying the impregnated catalyst can then be calcined to convert the deposited molybdate salt to the oxide. The carrier can then be treated with an aqueous solution of the Group VIII metal salt, followed by calcining. If a second iron group metal is to be employed, the second iron group metal can be deposited in a like manner. Nitrates or acetates of the Group VIII metals are normally utilized although any water soluble salt which leaves no harmful residue can be employed.
If desired, the Group VIII metal and the Group VI metal can be deposited simultaneously, but are preferably deposited in sequence with intervening calcining. Simultaneous impregnation of the Group VIII metals has been found to be satisfactory. Following impregnation of the support with the Group VI and Group VIII metals, the Group IV-B metal can be deposited on the support from an aqueous solution of the salt of the Group IV-B metal in the manner previously described. The Group VIII and IV-B metals can also be deposited simultaneously.
The operating conditions employed in the process of our invention comprise a temperature in the range from about 500.degree. to about 1000.degree. F. (260.degree. to 538.degree. C.), preferably in the range from about 600.degree. to about 800.degree. F. (316.degree. to 422.degree. C.) and more preferably in the range from about 650.degree. to about 800.degree. F. (344.degree. to 422.degree. C.). The space velocity can be in the range from about 0.25 to about 10.0 volumes of charge stock per volume of catalyst per hour and preferably is in the range from about 0.5 to about 5.0. The hydrogen feed rate employed ranges from about 500 to about 10,000 standard cubic feet per barrel of feed stock (14,160 to 283,200 liters per 159 liters), preferably is in the range from about 1,000 to about 8,000 SCF/B (28,320 to 226,560 liters per 159 liters), and more preferably is in the range from about 2,000 to about 6,000 SCF/B (56,640 to 169,920 liters per 159 liters). The pressure employed in the process of our invention can be in the range from about 100 to about 5,000 PSIG (7.02 to 352 Kgs/cm.sup.2). When treating a distillate charge stock, i.e., one containing substantially no residual components, the above-mentioned broad range is satisfactory. Preferably, however, we employ pressures in the range from about 500 to about 3,000 PSIG (35.2 to 211 Kgs/cm.sup.2). When treating a residual-containing stock, such as, for example, a reduced crude (atmospheric tower bottoms) or a residual stock boiling above about 950.degree. F. or 509.degree. C. (a vacuum tower bottoms), we prefer to employ pressures in the range from about 250 to about 2,500 PSIG (17.6 to 176 Kgs/cm.sup.2) and more preferably employ pressures less than about 2,000 PSIG (141 Kgs/cm.sup.2). We have also found that pressures below about 1,000 PSIG (70.6 Kgs/cm.sup.2) and even below about 800 PSIG (56.4 Kgs/cm.sup.2) can be employed satisfactorily to treat residual containing stocks in accordance with our process. This ability to employ comparatively low pressures when treating residual-containing stocks in accordance with our invention, provides several advantages. As will readily be understood, the use of lower pressures permits the use of less expensive reaction vessels. Surprisingly, however, we have found that the catalysts employed in our process age extremely well in the low pressure treatment of residual containing stocks.
In order to illustrate our invention in greater detail, reference is made to the following examples.